The most important section of the methanol synthesis process is the methanol reactor. As the synthesis reaction is strongly exothermic, heat removal is an important process. High average heat flux leads to fewer tubes and thus reduced costs.
As the methanol reaction is exothermic, the primary task of the reactor is to control the temperature. The reactor technologies that have been used extensively in commercial settings fall into two categories: multiple catalyst bed reactors and single bed converters.
The multiple catalyst bed reactors control the reaction temperature by separating the catalyst mass into several sections with cooling devices placed between the sections. Bed sizes are generally designed to allow the reaction to go to equilibrium.
The Haldor Topsoe collect, mix, distribute convertor is such a multiple catalyst bed reactor. This reactor has catalyst beds separated by support beams. The gas that is leaving the upstream catalyst is then collected and mixed with a quench gas for cooling. The mixed gas stream is evenly spread over the downstream catalyst bed. The reaction temperature is lowered and the conversion per pass rate is increased.
Another type of multiple catalyst bed reactors are the adiabatic reactors in series. Each catalyst layer is accommodated in a separate reactor vessel with intercoolers between each reactor. The feed gas is fed directly into the first reactor which increases the kinetic driving force for the reaction. This leads to a reduced catalyst volume compared to a quench type reactor.
A further type of multiple catalyst bed reactors are multi-stage radial flow reactors with intermediate cooling. Indirect cooling keeps the temperature close to the path of the maximum reaction rate curve (when the methanol concentration is plotted against temperature). Maximum, or close to maximum, conversion per pass is then achieved.
Whether the multiple beds are separated by structures and cooling equipment within the reactor or by separate reactors, the above reactors are expensive to construct. As an alternative, single bed reactors may be chosen, where heat is removed continuously from the reactor by transfer to a heat-removing medium. The reactor runs effectively as a heat exchanger.
In one design, a single bed reactor has helically-coiled tubes embedded in the catalyst bed. Compared to reactors with the catalyst inside the tubes, the heat transfer on the catalyst side is significant higher. As a result, material costs are saved since less cooling area is required.
An alternative design for the single bed reactor is much similar to a heat exchanger; it has a vertical shell and tube heat exchanger with fixed tube sheets. The catalyst in the tubes rests on a bed of inert material. Steam is generated by the heat of reaction and drawn off below the upper tube sheet. To achieve precise control of the reaction temperature, steam pressure control is applied. Operating at isothermal conditions enables high yields at low recycles. In addition the amount of by-products is minimized.
Yet a further alternative single bed reactor has double-tubes with catalyst packed between the inner and the outer tubes. The feed enters the inner tubes and is heated when flowing through the tubes. The gas then enters the space between the inner and the outer tubes and flows through the catalyst bed. In addition to being cooled by the gas in the inner tubes, the catalyst is also cooled by boiler water outside the double-tube. Since the catalyst bed temperature is hither near the inlet of the reactor and then lowers towards the outlet, the gas proceeds along the maximum reaction rate line. This means that a higher conversion per pass rate is achieved.
A single bed reactor which reduces the equipment cost is utilizing the redial flow principle. When designing a high capacity methanol converter, there are many potential advantages if the radial flow principle is utilized, especially a very small pressure drop. However if the flow is only limited to be directly from the centre to the outer perimeter (or vice versa) the flow velocity is very slow. This is problematic since it requires cooling tubes distributed very close which is mechanically challenging and expensive. Conventional radial converters tend to face problems with hotspots in the catalyst bed which to some extend can be limited by a higher flow velocity.
Known art offers little solution to this problem, as can be seen in the following references, where:
EP0359952A2 describes a system for the improvement in situ of conventional reactors for the synthesis of methanol. The catalytic mass is divided into several beds in series, each bed having a bottom and a conical diaphragm spaced from the free surface of the next catalytic bed in such a way as to create a space to the outer periphery of which the quench gas is fed so as to achieve in said space the optimum mixing with the partially reacted gas which has run axially through the upper catalytic bed, a tube is introduced below and central to the upper catalytic bed delimiting internally the catalytic mass of the lower beds from the upper bed; and the lower bed or beds with maximum pressure drop are transformed into a bed with a substantially radial flow by introducing two cylindrical walls coaxial with said tube substantially perforated and forming airspaces with the shell inner wall and with the tube outer wall respectively.
WO9964145 discloses methods for constructing packed-bed and monolith reactors/converters, which are more resilient against process disturbances than their conventional counterparts. These stabilized reactors have a reduced tendency to develop, in response to accidental or planned changes of operating parameters, transient hot spots which otherwise can compromise safe and economical reaction operation. The invention involves creating conditions under which transient heat waves that originate from the process disturbances propagate in different radial zones of the reactor with different speeds. As a result, they accumulate phase-shifts relative to each other and interfere destructively through intra-reactor radial heat flows. This constitutes the adaptive mechanism of suppression of the noxious high-temperature waves in exothermal reactors and affects their enhanced operational stability. The area of applicability of stabilized reactors includes chemical and petro-chemical industries as well as automotive (car catalytic converter), environmental (VOC incinerator) and power/heat generation (catalytic combustor) applications. Advantages of the SR are enhanced safety and life span of catalyst and other reactor components, and in production applications—improved throughput, selectivity and product quality.
EP1261419 describes a reactor of the staged adiabatic reactor type, which comprises at least one heat exchanger panel, preferably a printed circuit heat exchange panel, interposes between adiabatic beds of catalyst, wherein the facial area of the panels and the superficial facial area of the corresponding catalyst are substantially similar, and the panels include means defining discrete passages for handling of reactants and heat transfer media, wherein the means defining passages for heat transfer media provide for at least two differing flow path directions for the heat transfer media through the heat exchanger panel whereby the occurrence of temperature bias or differentials is reduced.
US2006171868 discloses a pseudo-isothermal radial chemical reactor for catalytic reactions, comprising a substantially cylindrical shell closed at the opposite ends by respective base plates, comprising a reaction zone in which a respective catalytic bed is supported and a plurality of heat exchangers placed in said reaction zone.
In the following, tubes shall be construed as enclosures of any circumferential shape, only characterized by being longer than the cross sectional distance. Typically tubes are cylindrical, but they may also have non-circular cross sectional shapes and varying cross sectional shape over the tube length.
Process fluid is defined as the process fluid (in any phase or mix of phases, gas, vapor or liquid) which is in the reactor, entering or leaving the reactor and is undergoing a reaction in the reactor. Whereas more specifically reactant is understood as the process fluid which is going to be or is being reacted and the product is the process fluid which has been reacted in the reactor. The limit where the process fluid is a reactant and where it is a product is floating, however when the process fluid is entering the reactor it is defined as being reactant and when it exits the reactor it is defined as being product.